#define F_CPU 8000000UL
#include <avr/io.h>

#define KPAD_PORT   PORTD
#define KPAD_DDR    DDRD
#define KPAD_PIN    PIND

/*
Column  PD0 - PD3
Rows    PD4 - PD7
             _____
        PC6 |     | pc5 (SCL)
  C0 -  PD0 |     | pc4 (SDA)
  C1 -  PD1 |     | pc3
  C2 -  PD2 |     | pc2 (ADC2)
  C3 -  PD3 |     | pc1 (ADC1)
  R0 -  PD4 |     | pc0 (ADC0)
        VCC |     | GND
        GND |     | AREF
        PB6 |     | AVCC
        PB7 |     | PB5 (SCK)
  R2 -  PD5 |     | PB4 (MISO)
  R3 -  PD6 |     | PB3 (MOSI)
  R4 -  PD7 |     | PB2 (SS)
        PB0 |_____| PB1


    Function return the key code when a key is pressed
    Keys are numbered as follows
    @param: None
    [01] [02] [03] [A]
    [04] [05] [06] [B]
    [07] [08] [09] [C]
    [*]  [0]  [#]  [D]
    Return 255 if none of the keys are pressed.
*/


uint8_t KeyPressed() {
    uint8_t row, colum;

    // The column C0 to C4 are kept at High Z State, these pins are not high or low, they are tristate. (inputs)
    // And their port value is set as low as well.
    // When the ddr (N)bit changes to 1, the port become output with value low.

    // 11110000
    // Enable pull-ups on the column's
    KPAD_PORT |= 0Xf0;

    for(colum=0; colum<4; colum++)
    {
        // Make all the pins inputs. (0)'s
        // The column inputs will have pullups as defined in KPAD_PORT
        KPAD_DDR &= ~(0Xff);
        _delay_ms(2);

        // Go though the colum's
        // Set the Column pin from High Z state to a output.
        // c=0       c=1       c=2       c=3
        // 1000nnnn, 0100nnnn, 0010nnnn, 0001nnnn
        KPAD_DDR |= (0X80>>colum);
        _delay_ms(2);

        for(row=0; row<4; row++)
        {
            // Go though each row.
            // r=0       r=1       r=2       r=3
            // nnnn1000, nnnn0100, nnnn0010, nnnn0001
            uint8_t pinvalue = KPAD_PIN & (0X08>>row);
            if(!(pinvalue))
            {
                // Return the button pressed!.
                return (row*4+colum);
            }
        }
    }
    return 0Xff;
}